I think really everyone should only worry about how their own character is built/played, unless someone asks for advice. This does also mean that the GM shouldn't force the party into situations where lack of optimization results in significant problems for the whole group.

As far as multiclassing and OP characters though - largely unconnected, IME. The most powerful classes in 3E are casters, and multiclassing is almost always a step down for them. Non-casters benefit a lot more, but even then it's more often pushing them from below-par to on-par rather than any kind of extreme power.